Twitter founder Evan Williams relinquished his position as CEO on Monday to Dick Costolo, the company’s COO. We’ve already examined Ev’s reasons for stepping down. “Ev is a visionary,” says Ryan Holmes, CEO of Hoot Suite, a third-party Twitter client. “Probably, on the day to day front, he wasn’t finding interest in doing his day to day duties.” That’s a fairly typical assessment of Ev’s motives.

But the question remains: who is this Dick? And what does he mean for the future of Twitter?

A University of Michigan graduate, Costolo is the founder of Feedburner, the RSS reader which was snapped up by Google in 2007. In addition to his dabblings in improv comedy, Costolo has held responsibility for Twitter’s business strategies since he joined the microblogging service in September 2009 (his first tweet upon joining: “Task #1: undermine CEO, consolidate power.”). He is the mastermind behind advertiser services like Promoted Tweets and Promoted Trends, both of which have been a boon for the company. (Promoted Tweets now sell for $100,000 each.)

All of this leads us to believe that Costolo was chosen for his ability to turn Twitter’s immense people-power into cash.Business Insider summarizes the three regimes of Twitter CEOs: Jack Dorsey defined Twitter as a product, Evan Williams scaled the service, and Dick Costolo will scale the business–assuming continued success, that is.

Scaling the business probably means that more advertisements will slowly creep into Twitter. The trick, though, will be to sneak in moneymaking schemes without turning off users who have become accustomed to a mostly ad-free service. If anyone can do it, Dick can.

With additional reporting by Austin Carr. Ariel Schwartz can be reached on Twitter or by email.AS